Visual effects

Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box, e.g.:

Whenever overflow occurs, the 'overflow' property specifies whether a box is clipped to its content box, and if so, whether a scrolling mechanism is provided to access any clipped out content.

11.1.1 Overflow: the 'overflow' property

This property specifies whether the content of a block-level element is clipped when it overflows the element's box (which is acting as a containing block for the content). Values have the following meanings:

Even if 'overflow' is set to 'visible', content may be clipped to a UA's document window by the native operating environment.

Consider the following example of a block quotation (BLOCKQUOTE) that is too big for its containing block (established by a DIV). Here is the source document:

<DIV>
<BLOCKQUOTE>
<P>I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.
<CITE>- Groucho Marx</CITE>
</BLOCKQUOTE>
</DIV>

Here is the style sheet controlling the sizes and style of the generated boxes:

div { width : 100px; height: 100px;
      border: thin solid red;
      }

blockquote   { width : 125px; height : 100px;
      margin-top: 50px; margin-left: 50px; 
      border: thin dashed black
      }

cite { display: block;
       text-align : right; 
       border: none
       }

The initial value of 'overflow' is 'visible', so the BLOCKQUOTE would be formatted without clipping, something like this:

Rendered overflow

Setting 'overflow' to 'hidden' for the DIV element, on the other hand, causes the BLOCKQUOTE to be clipped by the containing block:

Clipped overflow

A value of 'scroll' would tell UAs that support a visible scrolling mechanism to display one so that users could access the clipped content.

11.1.2 Clipping: the 'clip' property

A clipping region defines what portion of an element's border box is visible. By default, the clipping region has the same size and shape as the element's border box. However, the clipping region may be modified by the 'clip' property.

The 'clip' property applies only to absolutely positioned elements. Values have the following meanings:

The element's ancestors may also have clipping regions (e.g. if their 'overflow' property is not 'visible'); what is rendered is the intersection of the various clipping regions.

If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.

The following two rules:

p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }

will create the rectangular clipping regions delimited by the dashed lines in the following illustrations:

Two clipping regions

11.2 Visibility: the 'visibility' property

The 'visibility' property specifies whether the boxes generated by an element are rendered. Invisible boxes still affect layout (set the 'display' property to 'none' to suppress box generation altogether). Values have the following meanings:

This property may be used in conjunction with scripts to create dynamic effects.

In the following example, pressing either form button invokes a user-defined script function that causes the corresponding box to become visible and the other to be hidden. Since these boxes have the same size and position, the effect is that one replaces the other. (The script code is in a hypothetical script language. It may or may not have any effect in a CSS-capable UA.)

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<STYLE type="text/css">
<!--
   #container1 { position: absolute; 
                 top: 2in; left: 2in; width: 2in }
   #container2 { position: absolute; 
                 top: 2in; left: 2in; width: 2in;
                 visibility: hidden; }
-->
</STYLE>
</HEAD>
<BODY>
<P>Choose a suspect:</P>
<DIV id="container1">
   <IMG alt="Al Capone" 
        width="100" height="100" 
        src="suspect1.jpg">
   <P>Name: Al Capone</P>
   <P>Residence: Chicago</P>
</DIV>

<DIV id="container2">
   <IMG alt="Lucky Luciano" 
        width="100" height="100" 
        src="suspect2.jpg">
   <P>Name: Lucky Luciano</P>
   <P>Residence: New York</P>
</DIV>

<FORM method="post" 
      action="http://www.suspect.org/process-bums">
   <P>
   <INPUT name="Capone" type="button" 
          value="Capone" 
          onclick='show("container1");hide("container2")'>
   <INPUT name="Luciano" type="button" 
          value="Luciano" 
          onclick='show("container2");hide("container1")'>
</FORM>
</BODY>
</HTML>

Value:	visible \| hidden \| scroll \| auto \| inherit
Initial:	visible
Applies to:	block-level and replaced elements
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	as specified

Value:	<shape> \| auto \| inherit
Initial:	auto
Applies to:	absolutely positioned elements
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	For rectangle values, a rectangle consisting of four computed lengths; otherwise, as specified

Value:	visible \| hidden \| collapse \| inherit
Initial:	visble
Applies to:	all elements
Inherited:	yes
Percentages:	N/A
Media:	visual
Computed value:	as specified

11 Visual effects

11.1 Overflow and clipping

11.1.1 Overflow: the 'overflow' property

11.1.2 Clipping: the 'clip' property

11.2 Visibility: the 'visibility' property